One time I opened a 15th series wax box that had the white backs against the pack seal, so that all of the cards that were on the back of a pack had varying degrees of wax staining. Some were barely noticeable, others very noticeable. I started including a few in submissions to test the waters, starting with the least noticeable. Several got straight 9s. As the stains got more noticeable, several started getting 5s and 6s. I had requested no qualifiers, they otherwise would have gotten 8 or 9 (ST).
So a lot of it is the judgment of the grader as to how severe a defect is.